In a sputtering process, substrates that are to be sputter coated with a thin deposited film are positioned within a processing chamber adjacent to a sputtering target that is the source of the deposition material. The processing chamber is evacuated to provide a vacuum and the target has a negative voltage applied thereto. A plasma discharge is produced which creates positive ions that bombard the target separating atoms from the surface of the target. The atoms of target material travel to the substrate surface forming a coating of target material.
Targets are typically supported on a backing plate such that the back face of the target is against a corresponding surface of the backing plate. The target is typically firmly attached to the backing plate. The back surface of the target and the corresponding surface of the backing plate are typically highly polished in the belief that such highly polished surfaces aid in the heat transfer from the target to the backing plate.
Because the material of the target is used up in the sputtering process, sputtering sources are designed and built such that used targets can be removed and new targets mounted. To facilitate removal of the target, the target is typically clamped or bolted to the backing plate. The removal of a spent target and mounting of a new target is referred to as replenishment.
In the sputtering process, approximately 90% of the electrical power supplied is converted to heating of the target. The power densities applied to the target are of such high levels that aggressive cooling of the target is necessary to prevent melting or cracking of the target and to protect the sputtering structures from overheating.
Because of the high energy levels involved in sputtering, and the fact that a large portion of the energy is in the form of heat which must be dissipated, it is highly desirable to increase the heat transference from the target to the backing plate which, in turn, is cooled.
Cooling is commonly achieved by flowing coolant, typically water, against the backing plate. Heat is extracted from the target/backing plate assembly by conduction to the water.
In the past, efforts at cooling targets have been directed to thermal transfer by heat conductivity. To facilitate replenishment, the targets have been affixed to a backing plate, typically by clamps disposed around the periphery of the target plate and the backing plate.
By clamping or bolting the target to the backing plate, thermal conductivity occurs between the mating surfaces of the plates. In this arrangement heat transference occurs from the target to the backing plate. Various arrangements for conductive cooling of the backing plate are discussed in the prior art with a coolant fluid, most commonly water.
A heat transference limitation is that the surface of the target is not in complete contact with the mating surface of the backing plate. Also, sputtering operation occurs in a vacuum; and with no gas molecules to provide in convective transference between the gaps in the surfaces, not only is there limited conductive heat transference, there is no convective heat transfer in these areas.
Thus, there exists a need to enhance target thermal management to improve sputtering efficiency.